I truly thank
@Alex Wheeler and
@DocUSMCRetired for sharing their insight and experience.
I had a thought that I don't remember being discussed, that is when the shooter does not or can not control the load.
This applies to rimfire and anyone who can not or chooses not to handload.
My case point is my Weatherby Vanguard in 223. It is from the slow twist era around 2010. Before I got my reloading equipment, I went through every "light bullet" factory ammo eventually settling on Superformance 53g V-Max. Still struggled for precision. Then I got a barrel tuner. It worked as described on the tin. Once handloading, I tried other loads but duplicating that factory load with no change to the tuner worked.
I remember trying a limb saver on a 270 Weatherby Magnum with a pencil barrel and 12 twist. It sort of worked but I eventually abandoned it in favor of handload Barnes TTSX 95 going very fast and using "hunting simulation" of one cold + dirty followed by one follow up.
We know that barrel devices can be used to tune. It might not be the appropriate tool for the handloader but what about the non-handloader, what do you think?